The present invention relates to an apparatus for contacting two immiscible liquids (hereafter also referred to as fluids), more particularly usable in liquid-liquid extraction installations or to installations for the transfer of heat by direct contact. More specifically, it relates to apparatuses of this type in which the mixing and/or separation of the phases takes place by applying an electric field, the continuous field weakly conducting electricity.
In such apparatuses, the two immiscible phases are brought into cocurrent or countercurrent circulation and one of the phases is dispersed in the form of droplets or bubbles in the other phase, which constitutes the continuous phase. Electrodes arranged in an appropriate manner in the apparatus make it possible to apply an electric field to the fluid circulating there and to bring about, according to the field value applied, a coalescence of the droplets or bubbles of the dispersed phase or a mixture of the circulating fluids. Thus, when the electric field applied to the continuous phase is sufficiently intense, the electrostatic pressure which it exerts on the surface of the droplets exceeds the interfacial cohesion forces. The droplets explode, so that electrostatic sputtering or atomization is obtained. In this case, the field also makes it possible to increase the transfer coefficients.
However, when the electric field is weaker, it produces attraction forces between the droplets or modifies the movement thereof, thus increasing the probability of meeting, said different mechanisms accelerating the coalescence of the dispersed phase.
U.S. Pat. No. 4,161,439 illustrates an apparatus of this type in which a set of two electrodes is positioned with a variable spacing of the electrodes as a function of their height, so as to alternately produce in the column dispersion zones and coalescence zones of the droplets of the dispersed phase.
However, this apparatus suffers from the disadvantage of having a large cross-section compared with the useful passage cross-section of the phases. The circulation time and the residence time of the phases also differ in the dispersion and coalescence zones. Moreover, the electric supply system only makes it possible to vary the value of the field along the axis of the column and not its other characteristics. In the same way, the ratio between the fields of one stage and the other is fixed by the geometry of the electrodes and cannot vary.
British Pat. No. 1 205 562 illustrates an apparatus of the same type, in which use is also made of an electrostatic field for ensuring the dispersion and separation of the phases present.
When the dispersed phase is an aqueous phase having relatively high electrical conductivity, the electrodes can be covered with a layer of insulating material, e.g. polyethylene, to prevent any short-circuit between the electrodes to which high voltages are applied, cf the article of W. Kowalski and Z. Ziolkowski (International Chemical Engineering, vol. 21, no. 2, pp. 323-327).
However, the use of electrodes of this type does not make it possible to completely prevent in the long term the risks of short-circuits due to the breakdown of the insulant when using a highly electricity conducting dispersed phase with a high retention coefficient.